When an image such as fingerprint ridge lines is viewed in a small area, it can be considered as lines periodically spaced at equal intervals. Accordingly, such an image is divided into small regions, which can be regarded as periodic patterns, and frequency analysis such as Fourier transform is performed on each small region for coding or image enhancement.
There has heretofore been proposed a fingerprint/palmprint image processing system for performing frequency analysis on a fingerprint or a palmprint (hereinafter referred to as a fingerprint/palmprint).
For example, Japanese Unexamined Patent Publication No. 59-22173 (hereinafter referred to as Patent Document 1) discloses a “Fingerprint Image Coding Method” which reduces loads on pre-processing and post-processing for coding and facilitates fingerprint image coding. In the fingerprint image coding method disclosed by Patent Document 1, an entire fingerprint image is divided into small regions, Fourier transform is performed on each small region, and Fourier components of the small regions are coded so that the fingerprint image is coded.
Further, Japanese Unexamined Patent Publication No. 2002-99912 (hereinafter referred to as Patent Document 2) discloses an “Image Processing Apparatus” which removes noise from an input image such as a fingerprint image so as to improve image quality. In the image processing apparatus disclosed by Patent Document 2, block division means divides an input image into blocks, and Fourier transform means performs Fourier transform on an image of each block. Noise power reduction means estimates a power of the original image and a power of noise at respective frequencies, sets a gain to be smaller as relative sizes of these powers are larger, and multiplies an amplitude of Fourier component information at the corresponding frequency by the gain to thereby reduce noise power. Inverse Fourier transform means performs inverse Fourier transform on the Fourier component changed in amplitude, and image reconstruction means reads respective block images that have been subjected to the inverse Fourier transform altogether so as to restore the entire image. Specifically, in the image processing apparatus disclosed by Patent Document 2, an entire fingerprint image is divided into small regions, and Fourier transform is performed on each small region. Components representing ridge lines are selected from a power of the Fourier components in each region. The fingerprint image is reconstructed with use of the components selected in each region.
Japanese Unexamined Patent Publication No. 9-167230 (corresponding to U.S. Pat. No. 5,937,082) (hereinafter referred to as Patent Document 3) discloses a “Fingerprint/palmprint Image Processing Apparatus” which extracts a ridge line image accurately from a skin pattern image even if there are wrinkles having characteristics similar to those of ridge lines. In the fingerprint/palmprint image processing apparatus disclosed by Patent Document 3, a local information extracting unit extracts a plurality of ridge line candidate images from each of local regions into which an input image is divided. A first ridge line candidate image selecting unit selects a first ridge line candidate image that is a ridge line candidate image having a maximum likelihood of a ridge line from ridge line candidate images in each of the local regions. A connectivity evaluating unit evaluates a connectivity of the first ridge line candidate images between adjacent local regions, and local regions that have been evaluated to have a high connectivity therebetween are integrated by a clustering unit so as to generate a local region group. A cluster evaluating unit determines an initial local region group that is a local region group having a high likelihood of a ridge line. An optimum ridge line candidate image selecting unit selects a ridge line candidate image having a high connectivity with its surrounding from the ridge line candidate images in local regions that are not included in the initial local region group. A ridge line image restoring unit produces the ridge line candidate image with respect to the local regions included in the initial local region group and otherwise produces the optimum ridge line candidate image as a ridge line extraction result and results in using them as a ridge extract results.
Japanese Unexamined Patent Publication No. 2002-288641 (corresponding to U.S. Patent Application Publication No. 2002/0164056) (hereinafter referred to as Patent Document 4) discloses a “Fingerprint/palmprint Image Processing Apparatus” which can extract a ridge line more accurately even in a region in which wrinkles are mixed or at a portion in which the ridge line has a high curvature. In the fingerprint/palmprint image processing apparatus disclosed by Patent Document 4, a ridge line image that is supposed to show a ridge line is selected from ridge line candidate images in local regions other than highly reliable regions, and it is determined whether each selected ridge line image is effective as an image representing a ridge line. An entire image is generated based on the ridge line images in the highly reliable regions and the ridge line images that are determined as being effective in other regions.
Japanese unexamined Patent Publication No. 2002-288672 (corresponding to U.S. Patent Application Publication No. 2002/0164055) (hereinafter referred to as Patent Document 5) discloses a “Fingerprint/palmprint Image Processing Apparatus” which can extract a ridge line more accurately even in a region in which wrinkles are mixed or at a portion in which the ridge line has a high curvature. In the fingerprint/palmprint image processing apparatus disclosed by Patent Document 5, region feature judging means judges a region in which ridge lines singly exist or a region in which wrinkles and ridge lines are mixed. Suitable ridge line candidate selecting means selects images representing ridge lines from ridge line candidate images for the region in which wrinkles and ridge lines are mixed and for the region in which ridge lines singly exist, based on corresponding evaluation standards.
Specifically, in the fingerprint/palmprint image processing apparatus disclosed by Patent Document 3 to Patent Document 5, an entire fingerprint image is divided into small regions, and Fourier transform is performed on each small region. Components representing ridge lines are selected in each region based on the connectivity with Fourier components in adjacent regions. A fingerprint image is reconstructed with use of the components selected in each region.
However, in the fingerprint/palmprint image processing apparatus disclosed by Patent Document 1 to Patent Document 5, since two-dimensional Fourier transform is performed collectively on vertical and horizontal pixel values of small regions into which the entire fingerprint image is divided, the Fourier transform process cannot be started in a state such that input of the entire fingerprint image or the entire small region image has not been completed. As a result, it is inconvenient that an overall period of time required for the Fourier transform process becomes long because of a standby time for image input or the like.
A fingerprint image has heretofore been captured at a time with use of a sensor larger than a finger. In contrast thereto, there is a method to achieve a lower cost and a reduced size of an apparatus, which uses a sensor smaller than a finger and moves the finger and the sensor relative to each other (referred to as sweep operation) so as to acquire an entire image of the finger. For example, Japanese Unexamined Patent Publication No. 10-91769 (corresponding to U.S. Pat. No. 6,289,114) (hereinafter referred to as Patent Document 6) discloses a “Fingerprint Reading System” which can recognize and determine the best image with sufficient accuracy and high reliability at low cost. The fingerprint reading system disclosed by Patent Document 6 has reading means for reading a fingerprint while a sensor attached to the reading means and a finger are slid so as to provide relative movement therebetween in a state such that the finger is brought into contact with the sensor, and means for reconstructing an image of the fingerprint from partial images obtained during the movement.
However, in a case where a fingerprint image is to be inputted with use of such a sensor, since the fingerprint/palmprint image processing apparatus of Patent Document 1 to Patent Document 5 has a structure in which two-dimensional Fourier transform is performed collectively on vertical and horizontal pixel values, it is necessary to form an image of the entire fingerprint or the entire small region in advance with application of technology disclosed by Patent Document 6 in order to start the two-dimensional Fourier transform. As a result, there are problems that a period of time required for the process becomes redundant and that input means has a limitation in structure.
Further, Fourier transform should be performed on a sufficiently large area such that the periodicity of ridge lines is clearly recognized in each region. However, because coding or image enhancement is performed on each small region, the coding or image enhancement cannot fully be performed if the size of a small region is made larger, thereby causing an adverse influence. Accordingly, it is desirable that a small region is not made larger than necessary and that Fourier transform is performed on a wide area around the small region so as to guarantee the fineness of coding or image enhancement and improve the stability of the frequency analysis.
For example, in the embodiment disclosed by Patent Document 3, in order to calculate a frequency component of a small region of 8-pixel square, Fourier transform is performed on an image of 64-pixel square around the small region. However, in this case, an area to be subjected to Fourier transform becomes large. Particularly in a case where small regions are successively extracted so that a frequency component of each small region is calculated, two-dimensional Fourier transform should be performed on an image of 64-pixel square for each small region, so that a period of time required for arithmetic processing becomes extremely redundant.
In this manner, the technical idea that Fourier transform is performed on a wide area around a small region so as to guarantee the fineness of coding or image enhancement and improve the stability of a frequency analysis can be diverted not only to the fingerprint/palmprint image processing apparatus disclosed by Patent Document 3, but also, for example, to any fingerprint/palmprint image processing apparatus disclosed by Patent Documents 1, 2, and 3 to 5. However, the redundancy of a standby time for image input or a period of time required for arithmetic processing causes a bottleneck.
Further, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2001-511569 (corresponding to U.S. Pat. No. 5,953,442) (hereinafter referred to as Patent Document 7) discloses a method and an apparatus for automatically positioning a first unknown image such as an unknown fingerprint image into one of a plurality of categories. The invention disclosed by Patent Document 7 includes a step of storing a set of plural values derived from frequency images of the respective image categories into a library. A categorization method and apparatus take a frequency image of a first unknown pattern to create a first frequency image. A frequency image plane of the first (unknown) frequency image is divided into a plurality of frequency image plane regions. Each of the frequency image plane regions may be an angular segment radiating from the origin of the frequency image plane. A region value is assigned to each of the frequency image plane regions based on a total energy of the frequency image in the angular frequency image plane region. The region values for the first frequency image are combined to generate a first set of region values. The first set of region values is compared with each of the stored set of values in a comparator. The comparator preferably correlates the set or pattern of the region values with use of one-dimensional frequency transform on a spatial image of the pattern of the region value set.
In Patent Document 7, two-dimensional Fourier transform is performed collectively on pixel values of the fingerprint image. Accordingly, the method and apparatus disclosed by Patent Document 7 also have the same problems as the aforementioned fingerprint/palmprint image processing apparatus disclosed by Patent Document 1 to Patent Document 5.
Accordingly, an object of the present invention is to provide an image processing system, an image processing method, and an image processing program capable of reducing an overall processing time required for a frequency analysis-process.